Fast switching igbt with embedded emitter shorting contacts and method for making same

ABSTRACT

Integrated circuits are presented having high voltage IGBTs with integral emitter shorts and fabrication processes using wafer bonding or grown epitaxial silicon for controlled drift region thickness and fast switching speed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional patentapplication Ser. No. 13/611,653, filed Sep. 12, 2012, the contents ofwhich are herein incorporated by reference in its entirety

FIELD OF THE INVENTION

The present disclosure relates to the field of integrated circuits, andmore particularly to fast switching insulated gate bipolar transistor(IGBT) devices with embedded emitter shorting contacts.

BACKGROUND OF THE INVENTION

IGBTs include a bipolar transistor and a MOSFET. The bipolar emitter ison the bottom of the device (although in various descriptions, thebottom terminal is sometimes referred to as a “collector” where the IGBThigh-voltage terminal (the IGBT collector) is connected to the regionwhich functions as the emitter of the integral bipolar transistor), andoperates to inject minority carriers into the bipolar base, therebyfilling this region with a plasma of holes and electrons to facilitatehigh current density. Bipolar conduction in IGBTs provides an advantagein terms of current per unit area, but results in a disadvantage interms of switching speed. The excess carriers in the hole-electronplasma do not instantaneously disappear after the current has stoppedflowing, and the device cannot return to the off-state and support ahigh voltage with low leakage until the excess carriers are gone.Therefore, if the device is to be designed for fast switching, it isnecessary to build in a mechanism to provide rapid removal of excesscarriers.

Current is carried by both holes and electrons in bipolar devices havingan emitter region of one conductivity type adjacent a base region of theopposite conductivity type. During conduction, the emitter injects itsmajority carriers as minority carriers into the base region. Entry ofthese minority carriers into the base permits the entrance of equalquantities of base majority carriers, and thus the total carrierconcentration in the base region can rapidly exceed the base dopantconcentration. The result is conductivity modulation of the base region,in which the base conductivity becomes much higher, and resistivity muchlower, than the background value. This conductivity-modulated bipolarconduction advantageously permits the device to carry a much highercurrent density than a similar unipolar device. In an IGBT, the emitteroperates to emit carriers into a voltage supporting region at thebipolar base, and fast switching IGBTs can be built using emittershorting contacts connecting the emitter to the base for excess carrierremoval to turn the device off quickly. In general, a resistor or lowimpedance contact can be provided between the emitter and the base, inparallel with the emitter-base junction. This emitter-base shuntresistor may be connected externally, or may be built into thestructure.

The excess carriers can thus be removed quickly from base region tointerrupt the current flow for fast switching applications. One way todo this is to create recombination centers to provide mid-band energylevels where holes and electrons can recombine. Recombination centerscan be provided by doping the crystal with heavy metals, such as gold orplatinum, or by bombarding the crystal with high energy neutrons,protons, electrons, or gamma rays to produce localized damage sites.Shorted emitters have several advantages over recombination centers.Recombination centers are more effective at removing carriers at highcarrier densities than at low densities, while emitter shorts are moreeffective at low carrier densities, which is the condition duringswitching. As carrier density increases, more carriers encounterrecombination centers and recombine, but this limits the level ofconductivity modulation, and thereby increases the on-voltage. Emittershorts have better impact at low carrier densities. When current is lowenough that the voltage drop on the emitter-shorting resistor is lessthan the 0.6-0.8 volt built-in offset voltage of the junction, almostall majority carriers flow through the shorting contact or resistorrather than crossing the junction and injecting minority carriers. Withonly recombination centers, majority carriers continue crossing thejunction and injecting minority carriers even down to very low currentlevels, thereby slowing the device turn-off. With the emitter shorts,minority carrier injection ceases as soon as the drop across theshorting resistor falls below the 0.6-0.8 volt level. Emitter shortsthus reduce the low-current gain, but have only a small effect on highcurrent gain.

High voltage IGBT devices are used to switch high voltage electricalpower, and certain applications require fast switching times for bothturn-on and turn-off. For a given switching speed, an IGBT made withemitter shorts can have a lower on-voltage at both low-current andhigh-current levels than an IGBT made with only recombination centers.However, high voltage devices with high switching speeds require controlover the drift region thickness, and conventional techniques provide noway for backside processing to create emitter shorts for devices withfairly thin drift regions necessary to achieve high switching speeds.

SUMMARY OF THE INVENTION

Various aspects of the present disclosure are now summarized forcompliance with 37 CFR §1.73 to facilitate a basic understanding of thedisclosure by briefly indicating the nature and substance of thedisclosure, wherein this summary is not an extensive overview of thedisclosure, and is intended neither to identify certain elements of thedisclosure, nor to delineate the scope thereof. Rather, the primarypurpose of this summary is to present some concepts of the disclosure ina simplified form prior to the more detailed description that ispresented hereinafter, and this summary is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims.

Integrated circuits (ICs) and fabrication processes are provided formaking IGBTs with emitter shorting contact structures for fast switchingspeeds, and controlled drift region thickness using bonded wafers and/orepitaxially grown silicon. ICs are provided having IGBT emitters and oneor more associated shorting contacts formed proximate a joinderinterface of two bonded wafers or beneath an epitaxially grown silicondrift region. The drift region thickness is controlled in certainembodiments by selective material removal processing after MOS structureformation and/or by control of an epitaxial growth process. IGBTs arethus provided having relatively thin drift regions of about 100 μm orless in certain embodiments for achieving high switching speed operationby the provision of one or more emitter shorting contacts. The disclosedembodiments advantageously combine these two advantages in a manner notpreviously possible using conventional fabrication techniques.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings, in which:

FIG. 1 is a partial sectional side elevation view illustrating a bondedwafer integrated circuit with an insulated gate bipolar transistor(IGBT) having emitter shorting contacts formed at the bottom of an upperwafer according to one or more principles of the present disclosure;

FIG. 2 is a flow diagram illustrating an exemplary process for makingthe integrated circuit of FIG. 1;

FIGS. 3-9 are partial sectional side elevation views illustrating theintegrated circuit of FIG. 1, depicted in successive stages offabrication;

FIGS. 10 and 11 are partial sectional side elevation views illustratingfurther embodiments of a bonded wafer integrated circuit with an IGBThaving emitter shorting contacts formed in the top of a lower carrierwafer according to principles of the present disclosure;

FIG. 12 is a flow diagram illustrating an exemplary process for makingthe integrated circuit of FIGS. 10 and 11;

FIGS. 13-19 are partial sectional side elevation views illustrating theintegrated circuit of FIG. 10, depicted in successive stages offabrication;

FIG. 20 is a partial sectional side elevation view illustrating anotherembodiment of an integrated circuit with an IGBT having emitter shortingcontacts formed at the bottom of an upper wafer according to principlesof the present disclosure;

FIG. 21 is a flow diagram illustrating an exemplary process for makingthe integrated circuit of FIG. 20;

FIGS. 22-27 are partial sectional side elevation views illustrating theintegrated circuit of FIG. 20, depicted in successive stages offabrication;

FIGS. 28 and 29 are partial sectional side elevation views illustratingfurther integrated circuit embodiments including an IGBT with emittershorting contacts proximate the top of a carrier wafer according toprinciples of the present disclosure;

FIG. 30 is a flow diagram illustrating an exemplary process for makingthe integrated circuits of FIGS. 28 and 29; and

FIGS. 31-35 are partial sectional side elevation views illustrating theintegrated circuit of FIG. 28, depicted in successive stages offabrication.

DETAILED DESCRIPTION

One or more embodiments or implementations are hereinafter described inconjunction with the drawings, wherein like reference numerals are usedto refer to like or similar elements throughout. The various featuresare not necessarily drawn to scale and are provided merely to illustratethe various concepts of the present disclosure. Several aspects of theinvention are described below with reference to example applications forillustration. It should be understood that numerous specific details,relationships, and methods are set forth to provide a full understandingof the disclosed concepts. One skilled in the relevant art, however,will readily recognize that these concepts can be practiced without oneor more of the specific details or with other methods. In otherinstances, well-known structures or operations are not shown in detailto avoid obscuring the disclosed apparatus and processes, wherein thepresent disclosure is not limited by the illustrated ordering of acts orevents, as some acts may occur in different orders and/or concurrentlywith other acts or events. Furthermore, not all illustrated acts orevents are required to implement a methodology in accordance with thepresent disclosure.

High voltage IGBT devices are used to switch high voltage electricalpower, and certain applications require fast switching times for bothturn-on and turn-off. As seen in the several embodiments below, highvoltage IGBTs can be created by using wafer bonding and material removalprocessing and/or via epitaxial growth to facilitate the ability tocreate emitter shorts as well as to control drift region thickness forfast switching and to accommodate a given target breakdown voltagerating, for instance less than about 100 μm (e.g. about 40-50 μm infurther embodiments). These integrated circuit structures andfabrication processes represent a significant advance over conventionalsemiconductor manufacturing techniques which do not allow for back sideprocessing of a very thin wafer (e.g. 50-100 μm or less).

The inventors have appreciated that, particularly at low current levels,the described shorted emitter IGBTs have a lower on-voltage and fastturn-off times because the emitter short provides a path through whichcurrent can flow around the emitter junction instead of having to flowacross the 0.6-0.8 volt offset voltage of the junction. At high currentlevels, a emitter-shorted IGBT can have a lower on-voltage because itcan allow a higher level of conductivity modulation than can an IGBTwith only recombination centers. Several embodiments and differentexemplary fabrication processes are illustrated and describedhereinafter, some of which involve use of bonded wafer processingtechniques and some using epitaxial growth processing to facilitatedrift region dimensional control for fast switching speeds.

A first embodiment is illustrated and described in connection with FIGS.1-9 including an integrated circuit 100 with a shorted emitter IGBTdevice fabricated using wafer bonding techniques and selective backgrinding to provide a thin N− drift region (e.g., 40-50 μm) for highvoltage operation (e.g., >about 600 V) and high switching speed. In thisembodiment, P+ emitter regions 112 and one or more N+ shorting contactsare formed in an upper N− wafer prior to joinder with a carrier wafer.Further embodiments are illustrated with respect to FIGS. 10-19, inwhich P+ emitter regions 212 or N+ shorting contacts are formed insilicided trenches in an upper side of the carrier wafer prior to waferbonding, and the upper wafer thickness is reduced prior to MOS structureformation. FIGS. 20-27 depict another example with P+ emitter and N+shorting contacts formed in an upper N− wafer, followed by bonding to alower carrier wafer. MOS cell structures are thereafter built on/in anupper side of the upper wafer, after which the sacrificial carrier waferis back ground to expose the P+ emitters and N+ shorting contacts beforemetallization processing. Further embodiments are illustrated in FIGS.28-35 in which the emitters and shorting contact(s) are formed in theupper side of an N+ wafer, and N− epitaxial silicon is grown above theemitters and shorting contacts, with the MOS structures formed in thetop of the epitaxial layer prior to back grinding to expose the emittersand shorting contacts, followed by metallization processing.

Other implementations of these techniques can be used to provide otherrelatively thin drift region IGBT structures, for example, less thanabout 100 μm for any desired switching speed and breakdown voltage.Moreover, the upper wafer, lower carrier wafer, epitaxially grownsilicon, emitter structures, shorting contacts structures and/or the MOSstructures including source regions and body regions can be of a varietyof combinations of different conductivity types, wherein thecomplementary conductivity type combinations are also contemplated asfalling within the scope of the present disclosure. In this regard,integrated circuits are contemplated including IGBTs having PNP bipolardevices as illustrated, as well as implementations having NPN bipolartransistors with a base formed in a P type semiconductor body such as anupper wafer and/or epitaxial silicon, and the present disclosure is notlimited to the illustrated examples in which an N− upper wafer and/or Ntype epitaxial silicon is used.

Referring now to FIGS. 1-9, An integrated circuit (IC) 100 is shown inFIG. 1 with an insulated gate bipolar transistor (IGBT) including avertical PNP bipolar transistor 140 along with one or more lateral Nchannel MOSFET cells in the region of an upper or top side of the IC 100to control the base “b” of the bipolar transistor 140 to form an IGBT.Each of the MOSFET cells provides an N+ source zone or region 126 and aP body zone or region 124 arranged between the source zone 126 and adrift zone or voltage supporting region 111 having a thickness 142 ofless than about 100 μm in an N− semiconductor body 110 (e.g., N− siliconin certain embodiments). The drift region thickness 142 in certainembodiments is about 50 μm or less, for example, about 40-50 μm inspecific embodiments for enhanced switching speed. The drift regionthickness 142, moreover, can be set according to a desired breakdownvoltage rating for the resulting IGBT, for example, with about 10-20 Vper μm thickness being a general design guideline for silicon.

The IGBT may have any number of symmetrically disposed parallel P typebody regions 124 diffused into the upper region of the N− semiconductorbody 110. Each of the P diffusions 124 has an N+ source region 126 toform annular or striped channel regions within the respective Pdiffusion 124, where the channel regions are covered by a gate oxide 134which is, in turn, covered by a conductive polysilicon and/or metal gatecontact 130. An interlayer oxide or dielectric (ILD) 132 covers the topand sides of the gate 130. A conductive cathode electrode 136 isconnected to P+ regions 128 within the P+ body region diffusions 124 andto the N+ source regions 126. An IGBT anode connection 138 along abottom side 100B of the integrated circuit 100 establishes an emitterstructure including multiple P+ emitter regions 112 coupled to an N+carrier wafer structure 120 via a silicide layer 116 and an optionalpolysilicon layer 118, where the emitter regions 112 emit holes inoperation.

The IGBT in FIG. 1 also includes shorting contacts formed as N+ regions114 serving as emitter shorting contacts between the N− body 110 (thevertical bipolar base b) and the associated P+ emitter regions 112. Anynumber of one or more shorting contacts 114 can be provided, andmultiple contacts 114 are preferably interleaved with correspondingemitter regions 112 as shown, although not a strict requirement of allthe possible implementations of the concepts of present disclosure. Thecollector bipolar (cathode) connection is made by a conductivemetallization layer 136 shown at a top side 100T of the integratedcircuit 100 in FIG. 1 and the emitter (anode) is at the bottom 100B,which operates to emit holes to the N− region (bipolar base) in theIGBT. In the embodiment of FIG. 1, moreover, a silicide layer 116 isprovided in certain embodiments beneath the P+ emitter regions 124 andthe N+ emitter shorting contacts 126, and a thin polysilicon layer 118may be disposed between the silicide layer 116 and an N+ carrier wafer120 bonded along an interface to the polysilicon 118 during fabrication.The silicide layer 116 and/or the polysilicon layer 118 may be omittedin certain embodiments. A conductive anode contact layer 138 is formedat the bottom of the carrier wafer 120. External contacts for the gatestructures 130 are provided by further metallization structures (notshown) along the top side 100T, whereby the integrated circuit 100provides connectivity to the gate, collector (cathode) and emitter(anode) terminals of the resulting IGBT.

The IGBT structure of the integrated circuit 100 advantageously providesa voltage supporting region having a vertical dimension 142 extendingbetween the bottom of the P+ regions 128 and the tops of the P+ emitterregions 112 of less than about 100 μm in certain embodiments, about 50μm in certain embodiments, about 40-50 μm in further embodiments. Thiscontrolled dimension 142 of less than about 100 μm provides fasterswitching speeds and also sets the voltage breakdown rating for theIGBT, for example, greater than about 300 V for about 20-30 μm or more,about 600 V or more for about 40-50 μm thickness 142, about 1000 V ormore for about 100 μm or more in certain examples.

In operation, when a positive voltage is applied to gate electrode 130,the P type channel region of each cell inverts to connect the N+ sources126 to the N− body 110 of the drift region 111, which is the base of thePNP transistor 140 having P diffusions (collectors) 124. The P+ emitterregions 112 begin to inject holes into the N− region 110 to turn on thePNP transistor 140 over the full surface area of each cell. The deviceis turned off by removal of the signal to the gate 130, thereby removingthe base drive from region 110. The injected holes in the N− region 110are then removed and the bipolar transistor 140 turns off, withconduction through the shorting contacts 114 speeding the carrierremoval from the base for fast turn-off.

FIG. 2 illustrates an exemplary process 150 for manufacturing theintegrated circuit 100 of FIG. 1, and FIGS. 3-9 show the IC 100 atsuccessive stages of fabrication. Processing begins at 152 in FIG. 2with formation of a plurality of P+ emitter zones 112 at 152 in thelower side of an N− silicon wafer or other suitable upper semiconductorstructure 110. Any suitable process can be used at 152, for example,such as an implantation process 152 using an implant mask 153 asillustrated in FIG. 3. At 154 in FIG. 2, one or more N+ shortingcontacts 114 is/are formed in the lower side of the wafer 110, forinstance using a second implant mask 155 and corresponding implantationprocess as illustrated in FIG. 4. The individual shorting contacts 114are proximate one or more corresponding emitter zones 112 along thelower side of the wafer 110, wherein the relative sizing in the widthsof the emitters 112 and the shorting contacts 114 can be tailored tospecific applications. In addition, the implanted regions 112 and 114may be of any suitable depth, and need not be of the same depth. Incertain embodiments, the P+ and N+ regions 112 and 114 respectively formemitters 112 and shorting contacts 114 on the bottom surface of thewafer 110, and may be formed by implantation with dopants selected forbonding with a lightly doped N− wafer 110, such as using boron for theP+ emitter implants 112 and phosphorus for the N+ shorting contactimplants 114, both to a depth of about 1-5 μm at high dopingconcentrations, e.g., about 1e19 cm-3 in one possible implementation.Any suitable photolithographic processes may be used to form andsubsequently remove the implant masks 153 and 155, and any suitableadditional processing may be employed, such as for example, activationanneals following one or both of the implantations.

At 156 in FIG. 2, a silicide layer is formed over the emitter zones 112and the shorting contact regions 114 at the lower side of the upperwafer structure 110 using any suitable silicide formation techniques. Inone possible example, silicide is formed at 156 (e.g. Titanium SilicideTiSi₂, or Tungsten Silicide WSi_(x)) by chemical vapor deposition (CVD)using monosilane or dichlorosilane with tungsten hexafluoride as sourcegases, followed by annealing at 800-900° C. to create a conductivestoichiometric silicide layer 116 shown in FIG. 5. In another possibleimplementation, titanium or tungsten metal is sputter deposited onto thebottom of the implanted regions 112 and 114 and is then heated to acertain temperature (e.g., 800-900°) to react the silicon with thedeposited metal to form the silicide 116, preferably to a thickness ofabout several hundred angstroms. As further shown in FIG. 5, apolysilicon layer 118 may be optionally formed to any desired thicknessover the silicide 116 via a process 158, followed by an optionalannealing step (not shown). Also, a chemical mechanical polishing (CMP)process may be performed to create a smooth surface appropriate forwafer bonding, and hence the polysilicon layer 118 is preferably thin(e.g., a few hundreds of angstroms) following the CMP process to providea high conductivity wafer boding interface while providing some siliconto facilitate bonding with a silicon carrier wafer 120.

The polished polysilicon surface 118 of the N− wafer 110 is then bondedwith an N+ carrier wafer and the upper N− wafer 110 is thinned to thedesired thickness to establish the thin IGBT drift region dimension 142in FIG. 1 to facilitate fast switching operation. Following thisthinning, the upper side of the N− wafer 110 is used to manufacture MOSgate structures where the processing temperature is limited in certainembodiments to about 950-1000° C. to preserve the metallic properties ofthe embedded silicide layer 116 and final metallization of the top andbottom are performed to provide the IC structure 100 of FIG. 1.

At 160 in FIG. 2, a wafer bonding step is carried out to join a secondsemiconductor structure (sometimes referred to herein as a carrierwafer) 120 (FIG. 6) to the lower side of the wafer 110. Any suitablewafer bonding process can be employed at 160 to bond the polysiliconsurface 118 at the bottom of the N− wafer 110 to an N+ carrier wafer120. In certain embodiments, for example, a low-temperature hydrophobicbonding process can be used at 160 to pressure bond the structures 110and 120 together in a vacuum environment causing high-pressure,low-temperature bonding of silicon to silicon.

At 162 (FIGS. 2 and 7) a grinding or other material removal process isused to reduce the thickness of the N− structure 110 by removingmaterial from the upper side, while leaving a remaining thickness 122 ofabout 105 μm or less. As noted above, high switching speed isfacilitated by providing a drift region dimension 142 (FIG. 1) of lessthan about 100 μm, and the material removal process at 162 can establishthis drift region thickness 142 in consideration of the formation of theemitter regions 112 to a depth of about 1-5 μm in certain embodiments.Other suitable remaining thicknesses 122 can be used, such as about45-55 μm for embodiments in which a desired drift region thickness 142of about 40-50 μm is desired, where the grinding operation 162 ispreferably controlled in certain embodiments so as to set the finaldevice drift region depth 142 taking into account the emitter structurethickness. Moreover, multiple steps can be performed at 162, including amechanical back grinding operation followed by chemical mechanicalpolishing in certain embodiments.

After the material removal, the process 150 proceeds with formation at164 of one or more MOS cell structures on/in the upper side of the N−wafer 110, as seen in FIG. 8. In this regard, while the illustratedembodiments are shown as having horizontal channel MOSFET cells,vertical channel MOSFETs can be used in constructing an IGBT inalternative embodiments. Any suitable MOS processing steps may beemployed at 164 in order to form P type body zones 124 and N+ sourceregions or zones 126, where the P body zones 124 (and optional P+regions 128) are disposed between the N+ source zones 126 and the N−drift zone 111 in the remaining upper wafer 110. In one possiblesequence, P type dopants (e.g., boron) are implanted to form the bodyregions 124 to any suitable dopant concentration and depth/profile,followed by implantation of N type dopants (e.g., phosphorus) with highdopant concentration such as 1e19 cm-3 in the upper side of theremaining (e.g., thinned) wafer structure 110 to form the N+ sourceregions 126, using any suitable masking structures and steps, as well asthermal annealing steps as are known. In certain embodiments, moreover,the MOS processing temperature is limited, for example, to about950-1000° C. or less to preserve the metallic properties of any includedembedded silicide layer 116. Thereafter, MOS gate processing is employedto form a gate dielectric 134 and at least one gate contact structure130 proximate at least a portion of the source zone 126 which isinsulated with respect to the source zones 126 and the body zones 124.Additional ILD processing is performed to create the dielectric 132.Thereafter, the top and bottom metallization processes are performed at166 and 168, as seen in FIG. 9 in order to provide an upper source metalstructure 136 providing a cathode connection to the source regions 126and a separate metallization contact structure (not shown) for the gate130 (e.g., FIG. 1), as well as a bottom (anode) metal layer 138 formedalong the bottom of the N+ carrier wafer 120 for external connection tothe IGBT emitters 112.

Referring now to FIGS. 10-19, FIG. 10 illustrates a second integratedcircuit embodiment 202 with upper MOS cell structures and correspondingmetallization formed at the top of a thinned N-silicon wafer structure110 including an IGBT with various structures and operation as describedabove in connection with FIG. 1. The IC 202 in FIG. 10 includes a lowerstructure formed using an N+ carrier wafer 120 including trenches at thetop of the carrier wafer 120 filled with P+ polysilicon 212 overlyingsilicide regions 216 at the bottom of the trenches to form alternatingemitter structures 212 with interleaved N+ collector shorting contactregions 214 of the carrier 120, along with a bottom (anode)metallization contact layer 138. FIG. 11 illustrates a third ICembodiment 272 in which the lower structure is formed using a P+ carrierwafer 270 with upper trenches used to form N+ emitter shorting contacts284 overlying silicide structures 286 at the bottom of the trenches,where the intervening P+ portions of the carrier wafer 270 provide theIGBT emitters 282. In these embodiments, the P+ emitter regions 212(FIG. 10) or N+ shorting contacts 284 (FIG. 11) are formed in thesilicided trenches on the upper side of the carrier wafer 120, 270 priorto wafer bonding, and the thickness of the bonded upper wafer 110 isthen reduced prior to MOS structure formation. In this manner, the ICs202, 272 can be provided with a controlled drift region thickness 142less than about 100 μm in certain embodiments, about 50 μm in certainembodiments, about 40-50 μm in further embodiments to facilitate highswitching speeds for the resulting IGBT. Moreover, the wafer bondingprovides sufficient structure thickness to facilitate the MOS cellprocessing.

The second and third embodiments of FIGS. 10 and 11 each employdeposition of silicide into the bottom of shallow trenches, followed byfilling the trenches with polysilicon and a CMP process to at leastpartially reveal the original silicon surface of the N+ carrier wafer120 (FIG. 10) or of the P+ wafer 270 (FIG. 11). This approach mayadvantageously increase the silicon-to-silicon bonding strength of theinterface joinder of the N− upper wafer 110 and the carrier wafer 120,270. As seen in the IC embodiment 202 of FIG. 10, moreover, the carrierwafer 120 has an N+ doping, and the silicided trenches are locallylimited to the contact area with the emitter P+ collector structures212. The silicide 216 makes an electrical short between thecorresponding P+ emitter 212 and the adjacent portions of the N+ carrierwafer 120. Thus, the embodiments of FIGS. 10 and 11 do not requiresilicide across the entire wafer as in certain implementations of the IC100 of FIG. 1 above, and also do not require implantation processing toform the IGBT emitter's 212, 282 or shorting contacts 214, 284.

FIG. 12 illustrates a fabrication process 250 which can be used tomanufacture the integrated circuits 202, 272 of FIGS. 10 and 11, andFIGS. 13-19 illustrate fabrication of the IC 202 of FIG. 10 (using an N+carrier wafer 120), where similar/complementary processing can be usedto form the alternate embodiment 272 of FIG. 11 with a P+ carrier 270.The process 250 in FIG. 12 begins at 252 with formation of a pluralityof trenches (e.g., trenches 251 in FIG. 13) in an upper side of the N−carrier wafer structure 120 (P+ carrier wafer 270 would be used infabricating the IC 272 of FIG. 11). Any suitable trench formationtechniques may be employed to provide the trenches 251. For instance, anetch mask 253 may be formed as shown in FIG. 13 using suitabledeposition, exposure, development, cleaning techniques, etc., followedby an etch process to selectively remove portions of the upper side ofthe N+ carrier wafer 120 to form the trenches 251 to any suitabledepths, such as about 1-5 μm in one embodiment.

At 254 in FIG. 12, silicide is formed in the trenches, such as formationof silicide layers 216 in FIG. 14 at the bottoms of the trenches 251.Any suitable processing can be performed at 254 to form the silicide216. In one possible example, nitride is deposited over the upper sideof the carrier wafer 120 after formation of the trenches 251, and thenitride is etched using an isotropic nitride etch process (not shown)leaving nitride on the sidewalls of the trenches 251 while removing thenitride from the trench bottoms. Thereafter, titanium or tungsten orother suitable metal is deposited, such as by sputtering, followed byheating to a certain temperature (e.g., 800-900° C.) to initiate areaction to create the silicide 216 in the bottoms of the trenches 251where the nitride has been removed. The silicide 216, moreover, can beformed to any suitable thickness, such as about several hundredangstroms on the trench bottoms in one embodiment, where a certainamount of silicide may, but need not be, present along all or portionsof the trench sidewalls. Thereafter, any remaining unreacted metal andthe nitride are removed by suitable cleaning steps, to leave thesilicide 216 in the trench bottoms as shown in FIG. 13.

Thereafter at 256 (FIG. 12), P+ polysilicon is formed over the silicidelayers 216 in the trenches 251, as seen in FIG. 15 (N+ polysilicon wouldbe used for the IC embodiment 272 shown in FIG. 11). The polysiliconformation at 256 may continue to the point where the P+ polysiliconextends above the tops of the trenches 251, and a CMP or other materialremoval process can thus be performed at 258 (FIG. 15) to provide asmooth upper surface exposing portions 214 of the original N+ carrierwafer 120 between the P+ polysilicon-filled trenches 212.

As seen in FIG. 16, a wafer bonding process 260 is used to join thelower side of an N− upper wafer 110 to the upper side of the N+ carrierwafer 120, wherein any suitable joining process can be used, such as thepreviously described low-temperature hydrophobic bonding process (e.g.,at 160 in FIG. 2 above).

At 262 in the process 250 of FIG. 12, the thickness of the N− upperwafer 110 is reduced by removing a portion of the upper side to leave aremaining upper wafer thickness 222 in FIG. 17, such as about 100 μm orless in certain embodiments. Any suitable grinding, CMP or othermaterial removal processing can be employed at 262, such as thosedescribed at 162 in FIG. 2 above. The upper wafer thickness 222 can betailored to a given switching speed target, to thereby set the finaldevice drift region thickness 142 as discussed above in connection withFIGS. 10 and 11 (e.g., to provide a drift region thickness 142 less thanabout 100 μm in certain embodiments, about 50 μm in certain embodiments,about 40-50 μm in further embodiments).

At 264, the MOS cell structures (FIG. 18) are formed in the upper sideof the remaining N− wafer 110 to create N+ source zones 126, P bodyzones 124, P+ regions 128 and the gate structures including the gatecontact 130 by any suitable processes, such as those described above at164 in FIG. 2. Thereafter, top and bottom metallization processing isperformed at 266 and 268, respectively, as seen in FIG. 19 (e.g., whichmay use processing as described at 166 and 168 in connection with FIG. 2above). In this regard, the MOS processing at 264 and subsequentmetallization processing may in certain embodiments be limited to around950-1000° C. to avoid damaging the silicide 216 in the trench bottoms.

Referring now to FIGS. 20-27, a fourth embodiment of an integratedcircuit 302 is illustrated (FIG. 20) in which P+ emitters 112 and N+shorting contacts 114 are formed in an upper N− wafer 110, followed bybonding to a sacrificial lower carrier wafer 120 (not shown in FIG. 20).MOS structures are then built in/on an upper side of the upper wafer110, followed by back grinding to expose the emitters 112 and shortingcontacts 114 before metallization processing. As seen in FIG. 20, theresulting IC 302 provides an IGBT structure as generally described withrespect to the IC 100 in FIG. 1 with an anode metal layer 138 formedalong the lower side of the upper wafer 110 with the emitter zones 112extending between the drift zone 111 and the anode metal layer 138. Thisapproach advantageously avoids the use of silicide, and thereby permitsuse of higher temperature MOS processing in forming the source, gate andbody regions.

FIG. 21 depicts a process 350 for making the IC 302, with FIGS. 22-27further illustrating various stages of the fabrication process 350. Thefabrication begins at 352 in FIG. 21 with formation of P+ emitterregions 112, and formation of N+ emitter shorting contacts 114 alongsidethe emitters 112 on the lower side of the N− wafer 110 at 354. Anysuitable processing techniques can be used to form the emitter andshorting contact regions 112, 114, for example, the implantationprocesses illustrated and described above in connection with steps 152and 154 of FIGS. 3-5 above.

At 356 in FIG. 21, an optional polysilicon layer 118 (FIG. 22) is formedover the N+ emitter shorting contacts 114 and the P+ emitter regions 112on the lower side of the wafer 110, where the layer 118 may be formedusing any suitable techniques, such as described above at 158 in FIGS. 2and 5 (e.g., to a thickness of a few hundreds of angstroms in oneembodiment). As seen below, the polysilicon layer 118 facilitatesinitial bonding of a lower N+ carrier wafer 120, and thereafter may alsooperate as a material removal stop layer. A CMP process is thenperformed at 358 to provide a smooth lower surface as shown in FIG. 22.At 360, an upper side of an N+ carrier wafer 120 is bonded to the lowerside of the N− wafer 110 and any polysilicon layer 118 as shown in FIG.23, for instance, using the wafer bonding processes described above.

At 362, one or more material removal processes are performed, such asback grinding, CMP, etc., to remove material from the upper side of theN− wafer 110 as seen in FIG. 24. As in the above embodiments, theprocessing at 362 sets a remaining thickness 122 for the N− wafer 110 ofabout 105 μm or less in certain embodiments, which can be controlled tosmaller dimensions in order to set a capability switching speed bycontrolling the depth 142 (e.g., 100 μm or less) of the drift region 111(FIG. 20) of the resulting IGBT in the IC 302. MOS cell structures arethen formed at 364 (FIG. 25) on/in the upper side of the N− wafer 110,wherein the above-described MOS cell fabrication techniques can be used,without the temperature limitations associated with embodiments havingsilicide layers or regions.

Another material removal processes performed at 365 (shown in FIG. 26)to remove substantially all of the lower side of the N+ carrier wafer120, for example, stopping on the polysilicon layer 118. In this regard,the carrier wafer 120 may be entirely sacrificial, or a portion thereofmay remain in various embodiments. Any suitable material removaltechnique or processes may be employed at 365, such as those describedabove. At 366 and 388, top and bottom metallization processes areperformed as seen in FIG. 27 to provide the resulting IC 302 shown inFIG. 20 with a drift layer thickness 142 facilitating high voltageoperation and with the emitter shorting contacts 114 facilitating fastIGBT switching operation.

Fifth and sixth embodiments 402 and 472 are shown in FIGS. 28 and 29,respectively, with FIG. 30 illustrating a process 450 for making the ICs402 or 472, and FIGS. 31-35 showing various intermediary fabricationstages of the process 450 to form the IC 402. In these embodiments,emitters 412 and shorting contacts 414 are formed in the upper side ofan N+ carrier structure 120 (a P+ carrier wafer 170 is used in the IC472 of FIG. 29), and N− epitaxial silicon 410 is grown above theemitters 412 and shorting contacts 414. Also, the MOS cell structuresare formed in/on the top of the epitaxial layer 410 prior to backgrinding the bottom side of the carrier wafer 120, 170 to expose thebottoms of the emitters 412 and shorting contacts 414, followed bymetallization processing. The resulting integrated circuits 402 and 472provide IGBT operation as described above, with the epitaxial siliconthickness controlling the depth 142 of the drift region 111 for highvoltage breakdown rating and with the shorting contacts 414 facilitatingfast switching operation.

The fabrication processing begins at 452 in the process 450 of FIG. 30by forming spaced P+ doped emitter regions 412 in an upper side of an N+carrier wafer 120 is seen in FIG. 31, for example, using an implantationmask 453 and a suitable implantation process, such as the abovedescribed implantation process at 152 in FIGS. 2 and 3. N+ emittershorting contact regions 414 are alternatively implanted at 452 into theP+ carrier wafer 170 for the IC embodiment 472 of FIG. 29. At 454, N−epitaxial silicon 410 is grown (FIG. 32) over the upper side of thecarrier wafer 120 to a thickness 422 of about 100 μm or less, whereinthe thickness 422 provided by the epitaxial growth processing at 454 canbe tailored to provide a desired final sickness 142 of the IGBT driftlayer 111 as in the above embodiments. In the alternate embodiment ofFIG. 29, N− epitaxial silicon 410 is also formed at 454 in FIG. 30.

At 456, MOS cell structures are fabricated (FIG. 33) on the upper sideof the N− epitaxial silicon 410 to form source zones 126, body zones124, gate structures including insulated gate electrode 130, forinstance, by processing as described above. The thickness of the carrierwafer 120 (or 170) is reduced at 458 using one or more material removalprocesses (e.g., as described above) to remove a portion of the lowerside thereof, stopping when the implanted P+ emitters 412 (or implantedN+ emitter shorting contacts 414) are exposed as shown in FIG. 34.Thereafter, top and bottom metallization processes are performed at 460and 462 as shown in FIG. 35 to provide the finished IC 402. Theseembodiments provide a trade-off between the expense of growing theepitaxial silicon layer 410 and the use of wafer bonding techniques asdescribed above.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been disclosed with respect to only one of multipleimplementations, such feature may be combined with one or more otherfeatures of other embodiments as may be desired and advantageous for anygiven or particular application. Also, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in the detailed description and/or in the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

What is claimed is:
 1. A method of forming an integrated circuit, themethod comprising: forming a plurality of emitter zones of a firstconductivity type proximate a lower side of a first semiconductorstructure of a second conductivity type; forming at least one shortingcontact of the second conductivity type in the first semiconductorstructure proximate at least one of the of emitter zones proximate thelower side of a first semiconductor structure; joining a secondsemiconductor structure to the lower side of the first semiconductorstructure after forming the emitter zones and the at least one shortingcontact; reducing a thickness of the first semiconductor structure byremoving a portion of an upper side of the first semiconductor structureafter joining the first and second semiconductor structures; forming asource zone of the second conductivity type in the upper side of thefirst semiconductor structure after reducing the thickness of the firstsemiconductor structure; forming a body zone of the first conductivitytype in the upper side of the first semiconductor structure between thesource zone and a drift zone of the first semiconductor structure afterreducing the thickness of the first semiconductor structure; and forminga gate electrode proximate the upper side of the first semiconductorstructure proximate at least a portion of the source zone and insulatedwith respect to the source zone and the body zone.
 2. The method ofclaim 1, comprising forming a silicide layer on the lower side of afirst semiconductor structure after forming the emitter zones and the atleast one shorting contact and before joining the first and secondsemiconductor structures.
 3. The method of claim 1, comprising: forminga polysilicon layer on the lower side of a first semiconductor structureafter forming the emitter zones and the at least one shorting contactand before joining the first and second semiconductor structures; andremoving substantially all of the second semiconductor structure using amaterial removal process stopping on the polysilicon layer, afterforming the source zone, the body zone, and the gate electrode.
 4. Themethod of claim 1, wherein reducing the thickness of the firstsemiconductor structure comprises leaving a remaining thickness of thefirst semiconductor structure of about 105 μm or less.
 5. A method offorming an integrated circuit, the method comprising: forming aplurality of trenches in an upper side of a semiconductor carrierstructure; forming silicide layers in the plurality of trenches; formingpolysilicon over the silicide layers in the plurality of trenches;joining a second semiconductor structure of a second conductivity typeto an upper side of the semiconductor carrier structure after formingthe polysilicon over the silicide layers in the plurality of trenches;reducing a thickness of the second semiconductor structure by removing aportion of an upper side of the second semiconductor structure afterjoining the second semiconductor structure to the semiconductor carrierstructure; forming a source zone of the second conductivity type in theupper side of the first semiconductor structure after reducing thethickness of the second semiconductor structure; forming a body zone ofa first conductivity type in the upper side of the first semiconductorstructure between the source zone and a drift zone of the firstsemiconductor structure after reducing the thickness of the secondsemiconductor structure; and forming a gate electrode proximate theupper side of the first semiconductor structure proximate at least aportion of the source zone and insulated with respect to the source zoneand the body zone.
 6. The method of claim 5, wherein the semiconductorcarrier structure is of the second conductivity type; and whereinforming the polysilicon comprises forming polysilicon of the firstconductivity type over the silicide layers in the plurality of trenches.7. The method of claim 5, wherein the semiconductor carrier structure isof the first conductivity type; and wherein forming the polysiliconcomprises forming polysilicon of the second conductivity type over thesilicide layers in the plurality of trenches.
 8. The method of claim 5,wherein reducing the thickness of the second semiconductor structurecomprises leaving a remaining thickness of the second semiconductorstructure of about 100 μm or less.
 9. A method of forming an integratedcircuit, the method comprising: forming a plurality of spaced dopedregions of one conductivity type in an upper side of a semiconductorcarrier structure of a different conductivity type; forming epitaxialsilicon of a second conductivity type over the upper side of thesemiconductor carrier structure; forming a source zone of the secondconductivity type in an upper side of the epitaxial silicon; forming abody zone of a first conductivity type in the upper side of theepitaxial silicon between the source zone and a drift zone of theepitaxial silicon; forming a gate electrode proximate the upper side ofthe epitaxial silicon proximate at least a portion of the source zoneand insulated with respect to the source zone and the body zone; andreducing a thickness of the semiconductor carrier structure by removinga portion of a lower side of the semiconductor carrier structure afterforming the source zone, the body zone, and the gate electrode to exposethe plurality of spaced doped regions.
 10. The method of claim 9,wherein the plurality of spaced doped regions are of the firstconductivity type, and wherein the semiconductor carrier structure is ofthe second conductivity type.
 11. The method of claim 9, wherein theplurality of spaced doped regions are of the second conductivity type,and wherein the semiconductor carrier structure is of the firstconductivity type.
 12. The method of claim 9, wherein the epitaxialsilicon is formed to a thickness of about 100 μm or less.